The present invention relates to a bipolar transistor of the multi-emitter type, and more particularly to a bipolar transistor suitable for use for detection of a load current flowing in a load when the transistor performs by itself on/off control of current conduction to the load.
When on/off control of current conduction to a load is performed by a bipolar transistor, it is desirable that the transistor is arranged so that the transistor is protected from a thermal breakdown due to and overload current. Therefore, conventionally, such a bipolar transistor has been designed so as to have a circuit arrangement as shown in FIG. 1. That is, in FIG. 1, a bipolar transistor 1 is so arranged as to be turned on/off on the basis of the control output of a base control circuit 2. A load 3 is supplied with a current from a power source terminal 4 according to the transistor 1. Further, a current detection resistor 5 is provided in a current conduction path to the load 3, a current detection circuit 6 is provided for detecting a current flowing in the load 3 (and hence a collector current Ic of the transistor 1) by sampling a voltage across the resistor 5. The base control circuit 2 turns off the transistor 1 in the condition where the current detected by the current detection circuit 6 exceeds a predetermined upper limit value Imax, so that the collector current Ic of the transistor 1 is prevented from exceeding the upper limit value I.sub.max.
In such a configuration, the amount of heat generating in the resistor 5 increases corresponding to the load current Ic flowing in the transistor 1 and, therefore, not only a power loss in the resistor 5 increases but there is a possibility that the whole arrangement becomes large-scaled because it is necessary to use a large-scaled resistor having a large power capacity as the resistor 5. That is, for example, when the rated value of the load current Ic is 10 A and the value of the resistor 5 is 0.01 ohm, a voltage drop across the resistor 5 is 0.1 V and, accordingly, the resistor 5 is required to have a power capacity of 1 W at the lowest.